The present invention relates generally to intake and exhaust manifolds for internal combustion engines, and more specifically to manifolds providing improved power and torque over a wide range of engine speeds for multi-cylinder four-stroke internal combustion engines.
Intake manifolds of carbureted internal combustion engines transport combustion air and fuel, mixed in the carburetor, to the engine cylinders. Intake manifolds of fuel injected spark ignition and compression ignition (diesel) engines, which separately inject raw fuel close to or into the cylinders, transport combustion air only. Exhaust manifolds transport combustion products, called exhaust, from the cylinders to the atmosphere. A carbureted engine typically has its carburetor mounted over a plenum of the intake manifold. A fuel injected or diesel engine typically has an air intake mounted over a plenum of the intake manifold. Air, or an air/fuel mixture, enters the plenum and travels to the cylinders through ducts called pipes or runners. The runners exit at inlet ports at each cylinder. Inlet valves at each inlet port control the passage of air through the ports into the cylinders. Outlet ports, controlled by outlet valves, control the passage of exhaust to exhaust runners which lead to the atmosphere.
Intake air is drawn into each cylinder during an intake cycle (or stroke) primarily by the vacuum created by downward movement of the piston inside the cylinder. Exhaust is forced out of each cylinder during an exhaust cycle primarily by the pressure created by upward movement of the piston. The prior art has made substantial efforts toward increasing the amount and velocity of air, or air/fuel mixture, drawn into the cylinders during the intake cycle and increasing the amount of exhaust forced out of the cylinders during the exhaust cycle. Primarily, these efforts comprise supercharging the air, or air/fuel mixture, into the cylinders, and scavenging the exhaust out of the cylinders. Mechanical superchargers, driven off the camshaft or crankshaft, and turbochargers, driven by the force of exhaust gases, are used to force more air, or mixture, into the cylinders. Unfortunately, these mechanical devices add complexity and cost. Further, mechanical superchargers are inefficient at low engine speeds and turbochargers restrict the flow of exhaust. The prior art has sought to obtain the advantages of these mechanical add-on devices without their disadvantages by attempting to make "tuned" manifolds which utilize the pressure waves in the intake air and in the exhaust created by rapid piston movement to augment intake and exhaust by tuning the waves to be substantially in phase with the desired directions of movement of combustion air and exhaust.
Manifold tuning has been accomplished primarily by two methods, experimentation and mathematical modeling of manifold systems. In many cases a combination of the two methods is used. A conceptual model of the physical flows in a manifold is made, followed by experimentation on manifolds made in accordance with the teachings of the model.
An early model of intake manifold systems developed at MIT treats the runner to each cylinder as a quarter wave organ pipe resonator with internal acoustic vibrations. This model assumes steady state flow through the runner pipe. The acoustic model assumes a continuous wave in the runner that is initiated by intake valve closure and that the quarter wave organ pipe resonance frequency may be matched to the engine speed to improve performance. Experiments do not fully support the accuracy or usefulness of this model. One problem with the model is that it includes the invalid assumption that the intake valve is only open for 180 degrees of engine crank travel. A problem with using the model is that it requires a different manifold length correction factor for each engine speed for which one wishes to design. Another problem with using the model is that, used properly, it predicts useful runner lengths of approximately 200 inches for a typical automobile engine. Experiments using more practical runner lengths are measuring transients and other factors. Unfortunately, this model has achieved a popularity in use much greater than even its original developers and experimentors deemed supportable.
A later model of manifold systems tested at the University of Wisconsin placed a Helmholtz resonator at the carburetor inlet. Experimental manifolds attempting to take advantage of the teachings of this model have not achieved significant improvements in performance.
More complicated models have been proposed which require solving complicated differential equations, now made easier through the use of finite difference method and method of characteristics solutions on computers. These models generally assume a continuous wave model similar to the MIT model, but ignore the cylinder except as a source of a boundary condition which initiates a wave and produces a heat release due to the combustion in the cylinder. Attempts at experimental verification of these models in multi-cylinder engines require very time consuming model preparation and the solutions appear to be unique to each engine so that lessons learned from one engine may not be transfered to another. This model appears to give accurate estimates of engine power, fuel consumption and efficiency, but with the cumbersome long runners of the MIT model. Its usefulness for developing new manifolds for new engines is limited.
A recent flow model used in developing turbocharged exhaust systems assumes Fanno line (friction) flow as the basis for maximizing mass flow through the engine. Runner length is kept short to reduce losses and the runner is insulated between the exhaust valve and the turbine entrance. This model is not a resonance model and assumes that all waves are in phase.
The failure of the prior art single mode models led researchers at the University of Wisconsin, and later at The Ohio State University, to develop a two-mode model of manifold systems. This model is broken into two distinct parts or modes--as a Helmholtz resonator when the port valve is open, and as an organ pipe when the valve is closed. For the Helmholtz resonator model the cylinder is treated as the cavity and the runner as the neck. Experiments on single cylinder engines show the validity of this model. In a "short" (substantially less than one wave length) pipe manifold the Helmholtz model dominates and is used to time the pressure excursion that ram supercharges the intake or scavenges the exhaust.
Extension of the two-mode model from single to multi-cylinder engines changes the single degree-of-freedom equation of the single cylinder system into a two degree-of-freedom equation in multi-cylinder systems. The characteristic equation of the multi-cylinder system is developed by analogy to equations for parallel electrical L-C circuits, the electrical analog to a Helmholtz resonator. This produces a quadratic equation, the solution to which reveals two engine speeds at which performance will peak for both the intake and the exhaust manifold. Experiments show that the extended two-mode model usefully predicts manifold system performance in multi-cylinder engines.
A more complete description and analysis of the various models may be found in ASME (American Society of Mechanical Engineers) Paper No. 76-WA/DGP-4, "Short Pipe Manifold Design for Four-Stroke Engines," 1976; and, in ASME Paper No. 80-DGP-6, "Short Pipe Manifold Design for Four-Stroke Engines: Part II," 1980, both by the inventor, which are incorporated herein by reference. A specific discussion of the two-mode model may be found in ASME Paper No. 69-DGP-11, "The Two Types of Resonance in Intake Tuning," 1969, by Thompson and Engleman, also incorporated by reference. ASME Paper No. 76-WA/DGP-4 includes examples of using the equations developed for the two-mode two degree-of-freedom multi-cylinder model to design manifolds to produce the two separate torque peaks at preselected engine speeds.
Despite the valuable use in manifold design that may be made of the prior art teachings, the solutions are not simple to implement and require a high level of sophistication on the part of the manifold designer.
It is, therefore, a principal object of the present invention to provide a tuned manifold system that is simple to implement and provides a convenient starting point for designers wishing to further refine manifold tuning through experimentation.
It is another object of the present invention to provide a simple means for successfully implementing the advantages of the two-mode two degree-of-freedom model.
It is a further object of the present invention to provide a selection of manifold elements that may be combined to achieve specific desired engine characteristics.
A feature of the present invention is that the broadened power curve provided by two torque peaks allows the use of a transmission with fewer gears and thereby less complexity and reduced maintenance requirements.
An additional feature of the present invention is that it provides a free breathing system that adds power and performance at all engine speeds.
Yet another feature of the present invention is that its tuning is not affected by the additional use of superchargers or turbochargers.
An advantage of the present invention is that it increases power, improves emissions characteristics and reduces fuel consumption.
An additional and particular advantage of the present invention is that it lowers peak combustion temperature and thereby reduces thermal damage to engine components.
A further advantage of the present invention is that it reduces knock sensitivity.
Yet another advantage of the present invention is that it provides the advantages of ram supercharging without the complexity of mechanical add-on devices or of dual manifold systems having mechanical valves that switch from a low engine speed manifold system to a high engine speed manifold system.